The present invention relates to cooling methods and cooling apparatuses for a superconductor, and particularly to a method and an apparatus for cooling a superconductor such as an oxide superconductor having a high critical temperature.
Various refrigerants have been proposed for cooling a high temperature superconductor, while they cannot afford a complete satisfaction.
Liquid helium is expensive, having an extremely low specific heat at 4.2 K and thus being prone to become gas. The liquid helium used as a refrigerant for energizing does not provide good stability and is apt to generate resistance.
Liquid nitrogen is cheap and has a remarkably higher specific heat at 77 K as compared with that at 4.2 K, and the latent heat of the liquid is high. Therefore, the liquid nitrogen can be used to achieve effective cooling against a slight heat invasion and the stability when electric current is applied is high. However, due to the high boiling point of 77 K of the liquid nitrogen, conventional cables and magnets, which are designed to be cooled by the liquid nitrogen, cannot have a sufficiently high current density. The liquid nitrogen may be employed under a reduced pressure or a refrigerator may be employed to cool the liquid nitrogen, for example, in order to cool a high temperature superconductor below 77 K. However, nitrogen becomes a solid at 101,325 Nmxe2x88x922 (1 atm), 63.2 K. The solid nitrogen is relatively inferior in heat conduction and thus cooling efficiency could be impaired. In addition, cooling at approximately 63 K is not completely satisfactory. The critical current of a high temperature superconductor at 63 K could be about twice that at 77 K, but, consideration should be given to heat invasion to the superconductor. Even if any part of the superconductor reaches 63 K, the remaining part would attain a remarkably higher temperature than 63 K due to heat invasion. In this case, a part having the highest temperature limits the critical current of the entire superconductor.
Other refrigerants with its boiling point lower than 77 K include liquid hydrogen whose boiling point is 20 K. However, the hydrogen is explosive. Liquid neon having a boiling point of 27 K is a scarce material and thus very expensive, therefore, it is not generally applicable. Liquid oxygen has a relatively high boiling point of 90.2 K, while it has a relatively low freezing point of 54.3 K, and accordingly the liquid oxygen could be used together with a refrigerator for cooling it with the liquid form maintained. However, the liquid oxygen is explosive similarly to the liquid hydrogen. Additionally, a mixture of oxygen and nitrogen is proposed in Cryogenics 35 (1995) 387-391 as a refrigerant for cooling a high temperature superconductor at or below a liquid nitrogen temperature.
If a high temperature superconductor has a critical current density (Jc) of approximately 30,000 A/cm2 at the liquid nitrogen temperature, the occupying ratio of a superconducting wire in the cross section of a cable could be reduced and thus practical application could be expected. At present, however, a general high temperature superconducting wire has Jc of approximately 15,000 A/cm2. It is thus difficult by using such a wire to obtain a superconducting cable which is compact enough relative to the conventional normal conducting cable.
One object of the present invention is to provide a method and an apparatus by which a superconductor can be cooled to a lower temperature simply and conveniently at a low cost.
Another object of the present invention is to provide a method and an apparatus by which a large-scale high-Tc superconductor can be cooled safely at a lower cost.
The present invention is directed to a method of cooling a superconductor by a refrigerant, which includes the steps of cooling the refrigerant to or below a freezing point which is given by the refrigerant having a stationary state in the cooling system for the superconductor, maintaining the refrigerant system in a fluid state by a physical action on the refrigerant cooled to or below the freezing point, and cooling the superconductor by the refrigerant system in the fluid state to or below the critical temperature of the superconductor.
The method according to the present invention may further include the steps of measuring viscosity of the refrigerant and modulating flow rate of the refrigerant system in the fluid state according to the measured viscosity. The viscosity may be evaluated by the load applied to a stirring motor which is stirring the refrigerant.
In the method according to the present invention, the physical action on the refrigerant for maintaining the fluid state of the refrigerant system may be stirring of the refrigerant, transfer or circulation of the refrigerant via a pump, convection of the refrigerant in a vessel, or any combination thereof.
In a preferred embodiment of the present invention, the physical action on the refrigerant for maintaining the fluid state of the refrigerant system is stirring of the refrigerant in a vessel holding it and/or transfer or circulation of the refrigerant via a pump. The refrigerant may be cooled by a refrigerator or a refrigerating system, and the refrigerant system in the fluid state at or below the freezing point may be sent by the pump through a piping system from the vessel to a part in which the superconductor is housed.
In the present invention, the refrigerant may be selected from the group consisting of liquid nitrogen and a mixture of liquid nitrogen and solid nitrogen. The refrigerant may also be selected from the group consisting of liquid air and a mixture of liquid air and solid air. The refrigerant may further be selected from the group consisting of a mixture of liquid oxygen and liquid nitrogen and a mixture of liquid oxygen, liquid nitrogen and a solidified matter of at least one of the liquid oxygen and liquid nitrogen.
In the present invention, the superconductor may constitute at least one selected from the group consisting of an oxide high temperature superconducting cable, an oxide high temperature superconducting magnet, and an oxide high temperature superconducting device. If the superconductor constitutes the oxide high temperature superconducting cable, the step of cooling the refrigerant is preferably carried out at a plurality of places in the cooling system of the oxide high temperature superconducting cable.
In the present invention, the superconductor may be a part of a device selected from the group consisting of a transformer, a linear motor car, an SMES, an MRI, an SQUID, a logic circuit and a current limiter. In this case, the refrigerant can be sent from a cooling system independent of an operation unit of the device to the operation unit via a piping system.
The cooling method according to the present invention may further include the steps of liquefying air by a refrigerator, circulating the liquefied air while cooling it, and cooling the superconductor by the liquefied air which is cooled and circulated. The circulation of the liquefied air may be circulation by mechanical means such as a pump or the like, or convection of the liquefied air held in a vessel. The circulation may further be a cycle between vaporization of the liquefied air and condensation of the vaporized air by cooling it.
In the present invention, the refrigerator may employ a refrigerating cycle in which gas is compressed and expanded. The gas circulated in the refrigerating cycle may cool and condense air. Further, the gas expanded in the refrigerating cycle may cool the liquefied air which is circulated and cool the air for its condensation. The refrigerating cycle may be Brayton cycle. The gas used in the refrigerating cycle may be helium gas.
In the present invention, if the air is liquefied, a gaseous material which solidifies at a temperature higher than the liquefying temperature of the air is preferably removed from the air, and the air thus obtained is liquefied by the refrigerator. Any material for lowering the freezing point of the liquefied air may be added to the liquefied air. The material for lowering the freezing point may be petroleum-based organic solvent or zeolite. Liquid oxygen may be added to the liquefied air to use the resulting refrigerant mixture for cooling a superconductor. A cooling storage type refrigerator may be used to condense the air and/or cool the liquefied air which is circulated.
The present invention is further directed to an apparatus for cooling a superconductor by a refrigerant, which includes a refrigerating apparatus or a refrigerating system for cooling the refrigerant to or below a freezing point which is given by the refrigerant having a stationary state in the cooling system for the superconductor, means for allowing the cooled refrigerant to flow, means for measuring viscosity of the refrigerant, and means for modulating fluid state of the refrigerant according to the measured viscosity. The means for allowing the refrigerant to flow may be at least one selected from the group consisting of a pump and a stirring machine.
The cooling apparatus according to the present invention may further include a refrigerator having a cryogenic part exhibiting a temperature lower than the liquefying temperature of the air, a liquefied air storage vessel in which at least a part of the cryogenic part is housed, a first piping system for discharging the liquefied air stored in the vessel therefrom, a second piping system for directing the discharged liquefied air to the superconductor and circulating the liquefied air, and means placed at the second piping system for cooling the liquefied air supplied to the superconductor. A valve may be placed at the first piping system and a pump may be placed at the second piping system for feeding the liquefied air.
The refrigerator for cooling the air may employ a refrigerating cycle in which gas is circulated through its compression and expansion. In this case, a heat exchanger for cooling the liquefied air and/or the liquefied air storage vessel may be placed at the cryogenic part of the refrigerator through which the expanded gas passes. The refrigerating cycle may be Brayton cycle. The gas used in the refrigerating cycle may be helium gas. Preferably, a purifying unit is placed at a system for supplying air to the liquefied air storage vessel in order to remove from the air a gaseous material which solidifies at a temperature higher than the liquefying temperature of the air. A heat exchanging fin is preferably placed at the cryogenic part of the refrigerator housed in the liquefied air storage vessel. Preferably, a heater is placed at the cryogenic part of the refrigerator housed in the liquefied air storage vessel in order to melt or sublimate any solidified matter attaching to the cryogenic part. Further, an exhausting unit is connected to the liquefied air storage vessel for discharging the melt or sublimate. The cooling apparatus according to the present invention may further include means for injecting into the vessel an additive for lowering the freezing point of the liquefied air, and means for stirring the liquefied air containing the additive. Means for introducing liquid oxygen into the vessel may further be included. In order to condense the air in the vessel, a cooling stage of a cooling storage type refrigerator may be placed in the vessel, and a cooing stage of a cooling storage type refrigerator may be placed at the second piping system in order to cool the liquefied air.
The cooling apparatus according to the present invention may further include a refrigerator having a cryogenic part exhibiting a temperature lower than the liquefying temperature of the air, and a vessel in which at least a part of the cryogenic part and a superconductor to be cooled are simultaneously housed. The vessel holds the liquefied air for cooling the superconductor, and convection of the liquefied air occurs between the cryogenic part and the superconductor in the vessel or vaporization of the liquefied air and condensation of the vaporized air by the cryogenic part occur in the vessel. The refrigerator may be any of Brayton cycle type, Stirling type, GM type and Solvay type, or any combination thereof. The refrigerator may employ a refrigerating cycle in which gas is circulated through its compression and expansion. A heat exchanger for cooling the liquefied air may be placed at the cryogenic part of the refrigerator through which expanded gas passes. The cooling apparatus preferably includes a purifying unit for removing from the air a gaseous material which solidifies at a temperature higher than the liquefying temperature of the air. Preferably, a heat exchanging fin is placed at the cryogenic part in the vessel. A heater may be placed at the cryogenic part in the vessel, and an exhausting unit may be connected to the vessel. The cooling apparatus may further include means for injecting into the vessel an additive for lowering the freezing point of the liquefied air, and means for stirring the liquefied air containing the additive. The cooling apparatus may further include means for introducing liquid oxygen into the vessel.
The present invention is directed to a method of using liquefied air to cool a superconductor, which includes the steps of liquefying air by a refrigerator, circulating the liquefied air while cooling it, and cooling the superconductor by the liquefied air which is cooled and circulated. The circulation of the liquefied air may be generated by transfer by mechanical means such as a pump or the like, or generated by convection of the liquefied air held in a vessel. The circulation may be a cycle between vaporization of the liquefied air and condensation of the vaporized air by cooling it. The refrigerator may employ a refrigerating cycle in which gas is compressed and expanded. The gas circulated in the refrigerating cycle can cool and condense the air. The gas expanded in the refrigerating cycle may be used to cool the circulated liquefied air and cool the air to condense it. The refrigerating cycle may be Brayton cycle. The gas used in the refrigerating cycle may be helium gas. Preferably, the cooling method further includes the step of removing from air a gaseous material which solidifies at a temperature higher than the liquefying temperature of the air, and the resulting air is preferably liquefied by the refrigerator. A material for lowering the freezing point of the liquefied air may be added to the liquefied air. The material for lowering the freezing point may be petroleum-based organic solvent or zeolite. The cooling method may further include the step of adding liquid oxygen to the liquefied air, and the resulting refrigerant mixture is used for cooling a superconductor. By a cooling storage type refrigerator, air can be condensed and/or the circulated liquefied air can be cooled.
The present invention is directed to an apparatus using liquefied air to cool a superconductor, which includes a refrigerator having a cryogenic part exhibiting a temperature lower than liquefying temperature of the air, a liquefied air storage vessel in which at least a part of the cryogenic part is housed, a first piping system for discharging liquefied air stored in the vessel therefrom, a second piping system for directing the discharged liquefied air to the superconductor and circulating the liquefied air, and means placed at the second piping system for cooling the liquefied air supplied to the superconductor. In this apparatus, a valve may be placed at the first piping system and a pump may be placed at the second piping system for feeding the liquefied air. The refrigerator may employ a refrigerating cycle in which gas is circulated through its compression and expansion. Preferably, a heat exchanger for cooling the liquefied air and/or the liquefied air storage vessel are/is placed at the cryogenic part of the refrigerator through which the expanded gas passes. The refrigerating cycle may be Brayton cycle. The gas used in the refrigerating cycle may be helium gas. Preferably, a purifying unit is placed at a system for supplying air to the liquefied air storage vessel in order to remove from the air a gaseous material which solidifies at a temperature higher than the liquefying temperature of the air. Further, a heat exchanging fin is preferably placed at the cryogenic part of the refrigerator which is housed in the liquefied air storage vessel. A heater may further be placed at the cryogenic part of the refrigerator housed in the liquefied air storage vessel, and an exhausting unit may be connected to the liquefied air storage vessel. The cooling apparatus may further include means for injecting into the vessel an additive for lowering the freezing point of the liquefied air, and means for stirring the liquefied air containing the additive. The cooling apparatus may further include means for introducing liquid oxygen into the vessel. In order to condense the air in the vessel, a cooling stage of a cooling storage type refrigerator may be placed in the vessel, and a cooling stage of a cooling storage type refrigerator may be placed at the second piping system in order to cool the liquefied air.
Another cooling apparatus for a superconductor according to the present invention includes a refrigerator having a cryogenic part exhibiting a temperature lower than liquefying temperature of air, and a vessel which simultaneously houses at least a part of the cryogenic part and a superconductor to be cooled, wherein the vessel holds liquefied air for cooling the superconductor, and convection of the liquefied air occurs in the vessel between the cryogenic part and the superconductor or vaporization of the liquefied air and condensation of the vaporized air by the cryogenic part occur in the vessel. The refrigerator may be any of Brayton cycle type, Stirling type, GM type and Solvay type, or any combination thereof. The refrigerator may use a refrigerating cycle in which gas is circulated through its compression and expansion. A heat exchanger for cooling the liquefied air may be placed at the cryogenic part of the refrigerator through which expanded gas passes. Further, the cooling apparatus preferably includes a purifying unit for removing from the air a gaseous material which solidifies at a temperature higher than liquefying temperature of the air. A heat exchanging fin may be placed at the cryogenic part in the vessel. A heater may be placed at the cryogenic part in the vessel and an exhausting unit may be connected to the vessel. The cooling apparatus may further include means for injecting into the vessel an additive for lowering freezing point of the liquefied air, and means for stirring the liquefied air containing the additive. The cooling apparatus may further include means for introducing liquid oxygen into the vessel.